1. Field of the Invention
The present invention relates to switched reluctance motors for motor vehicles in general, and more particularly to an automatic control of the turn-on angle used to excite the switched-reluctance motor.
2. Description of the Prior Art
In the automotive industry there is rapid expansion in the incorporation of electronic and electrical systems for vehicle control, passenger comfort and safety, pollution reduction and economy of production, running and maintenance. The modern road vehicle relies heavily on electric motors (and drives); this dependency will inevitably increase, but requires an alternative to the convenient but costly a/c or d/c electrical motors. The current trend in electric motor control is to design low cost, highly energy efficient and highly (time) reliable systems.
Automotive electronic systems, particularly those associated with the engine compartment, must operate under difficult environmental conditions (temperature extremes, vibration, EMI, dirt mixed with oil, moisture and gas). It is necessary to cope with these conditions using good engineering practice and design.
The switched-reluctance motor (SRM) produces torque through excitation that is synchronized to rotor position. The simplest excitation strategy for the SRM is generally described by three excitation parameters: the turn-on angle xcex8on, the turn-off angle xcex8off, and the reference current Iref. A control algorithm would typically use the same excitation parameters for each phase, implemented with the spatial shift consistent with the symmetrically displaced phase structure. Control of the excitation angles results in either positive net torque for motoring, or negative net torque for generating. Basic operation of the SRM is given in several articles, such as xe2x80x9cVariable-speed switched reluctance motorsxe2x80x9d, P. J. Lawrenson, et al., IEE Proc., Vol. 127, pt. B, no. 4, pp. 253-265, 1980; xe2x80x9cSwitched Reluctance Motors and Their Controlxe2x80x9d, T. J. E. Miller, Oxford, 1993; and xe2x80x9cSwitched Reluctance Motor Drivesxe2x80x9d, R. Krishnan, CRC Press, 2001, which are hereby incorporated by reference.
Efficient operation of the SRM, or any motor drive, is always of importance. Inefficiency leads to larger size, increased weight, and increased energy consumption. In order to maximize SRM efficiency, the need exists to maximize the ratio of the average torque to RMS phase current, Tavg/Iphrms. This ratio captures the intended goal of providing the required electromechanical output with the minimum electrical input. This approach is valid for both drive applications that are tolerant of SRM torque ripple and applications that require extremely smooth torque production, though smooth torque production may require current shaping that cannot be characterized by the single parameter Iref.
While the self-tuning approach to optimization of excitation parameters, such as described by xe2x80x9cSelf-tuning control of switched reluctance motors for optimized torque per Ampere at all operating pointsxe2x80x9d B. Fahimi, et al., Proc. Of the IEEE Applied Power Electronics Conf., pp. 778-783, 1998; and xe2x80x9cA self-tuning controller for switched reluctance motorsxe2x80x9d, K. Russa, et al., IEEE Trans. On Power Electronics, Vol. 15, pp. 545-552, 2000; or the approach based on extensive lookup tables is know, this invention seeks to provide an automatic excitation angle control algorithm that supports efficient operation of the SRM over its entire speed region.
Thus, the need exists for a means to control the SRM that is simple, compact, inexpensive, providing better performance and efficiency, and greater reliability over the entire speed region of the SRM.
The present invention provides an improved control apparatus and method for a switched reluctance motor (SRM) for use with automobiles or other devices utilizing a switched reluctance motor.
Switched reluctance machines (SRM) are brushless d.c. machines, having neither brushes or permanent magnets, thereby minimizing potential maintenance and wear issues. The SRMs are durable and long lasting, with bearing life being the primary wear determinant. The SRMs are typically less expensive to manufacture due to fewer parts and less labor. The overall motor and drive system cost is largely a function of the cost of the electronic drive controller, depending on the level of sophistication required by the application.
Switched reluctance machines operate on the principle that a magnetic field that is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to become aligned with the magnetic flux (lines of force) generated by the magnetic field. Thus, by using the stator to establish and rotate a magnetic field about a rotor formed from magnetically permeable material, the rotor can be driven to rotate relative to the stator. The resistance to the passage of this magnetic flux from the stator to the rotor is referred to as reluctance. The magnitude of this reluctance changes with the rotational position of the rotor relative to the stator. Thus, electric motors of this type are commonly referred to as variable reluctance motors.
Typically, the SRM conventionally comprises a generally hollow cylindrical stator having a plurality of radially inwardly extending poles formed thereon, and a rotor rotatably supported concentrically within the stator and provided with a plurality of radially outwardly extending poles, i.e., SRM is doubly salient. Windings of an electrically conductive wire are provided about each stator pole. However, no electrical conductor windings or permanent magnets are provided on the rotor that consists only of iron laminations. Interconnecting the stator windings forms phase windings. For an SRM with q phases, the coil around every qth stator pole would be connected with alternating magnetic polarity whereby magnetic flux is alternately directed toward the rotor and away from the rotor. SRM phase windings are made with both series and parallel connections of stator coils, according to the intentions of the designer.
Torque is produced by switching current into each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. Thus, electric machines of this type are commonly referred to as switched reluctance machines. The current is switched off in each phase before the rotor poles nearest the stator poles of the phase rotate past the aligned position. Otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied to develop torque in either direction. These pulses are generated by a converter using current switching elements such as thyristors or transistors.
In operation, each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding of that phase providing energy from a direct current (DC) supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate toward a minimum reluctance configuration and partly in stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the DC source.
The switched reluctance machine can be utilized also as a generator. When operated as a generator, the SRM produces current rather than voltage. Braking torque is produced when winding current continues to flow after a rotor pole has passed alignment with an associated stator pole. Because the SRM has no rotor excitation, it is necessary to first draw electric power from a DC bus in order to cause current to begin flowing in windings of the motor. Current can be initiated in the windings either prior to alignment of a rotor pole and associated stator pole or after alignment has occurred. In general, very little torque will be produced by currents that exist when a corresponding rotor pole is adjacent or close to either side of a stator pole. Once the rotor pole passes alignment or continues into the negative torque region, the winding current will build faster than in the motoring region because the inductive term which establishes the voltage across the motor winding becomes negative. While some DC current will still be drawn from the associated DC bus while generating torque is being produced, DC current will be delivered to the bus when the switches actuated to start current into the winding are turned off and force the winding current to commutate into the associated flyback diodes. The net DC current is the sum of all the current from all of the phases of a multi-phase motor and it is this net DC current that is regulated when the reluctance motor is operated as a generator.
This invention presents a new approach to the automatic control of the turn-on angle used to excite the switched-reluctance motor (SRM). The control algorithm determines the turn-on angle that supports the most efficient operation of the motor drive system, and consists of two parts. The first part of the control technique monitors the position of the first peak of the phase current (xcex8p) and seeks to align this position with the angle where the inductance begins to increase (xcex8m). The second part of the controller monitors the peak phase current and advances the turn-on angle if the commanded reference current cannot be produced by the controller. The first part of the controller tends to be active below base speed of the SRM, where phase currents can be built easily by the inverter and xcex8p is relatively independent of xcex8m. The second part of the controller is active above base speed, where the peak of the phase currents tends to naturally occur at xcex8m, regardless of the current amplitude. The two parts of the controller naturally exchange responsibility as a result of a change in command or operating point.
Therefore, this invention provides an automatic excitation angle control algorithm that supports efficient operation of the SRM over its entire speed region and provides better performance and efficiency than the current electrical motors.